1. Field of the Invention
The invention is generally related to the field of moisture removal from gases and liquids. More specifically, the invention relates to apparatus adapted to reduce or remove moisture from gas-phase and liquid-phase acid gases, and methods of using same.
2. Related Art
Moisture is known to react with so-called xe2x80x9cacid gasesxe2x80x9d, such as hydrogen sulfide, carbonylsulfide, carbondlsulfide and mercaptans (mercaptans are also referred to as thiols) to form a complex compound. (The term xe2x80x9cacid gasxe2x80x9d is used herein to denote either gas phase, liquid phase, or mixture of gas and liquid phases, unless the phase is specifically mentioned.)
One problem presents itself: if one is interested in producing acid gas standard compositions, in other words acid gases having a known concentration of one of these gases in a matrix or carrier fluid, then one must consider how to reduce or remove the moisture. Gas standards may have to have, and preferably do have, a long shelf life, since the standard acid gas may not be required immediately after production. A source of acid gas may contain a considerable amount of moisture. Therefore, the reduction or removal of moisture from the acid gas is of primary importance if the stability of the acid gas in the standard gas is to be maintained.
Grossman et al. (U.S. Pat. No. 4,082,834) describes alloys, such as alloys of nickel, titanium, and zirconium, that react with water and reactive gases (such as hydrogen, hydrogen-containing compounds such as hydrocarbons, carbon monoxide, carbon dioxide, oxygen, and nitrogen) at temperatures ranging from about 200xc2x0 C. to about 650xc2x0 C. While the patent does not discuss acid gases, it is apparent that hydrogen sulfide, carbonyl sulfide, and mercaptans are hydrogen-containing compounds, so that there would not be any expected benefits using these alloys to remove moisture from these acid gases. While carbondisulfide does not contain hydrogen, and therefore there could be some moisture reduction from a composition comprising carbondisulfide and moisture using these alloys, the high temperature is prohibitive for commercial use.
Tamhankar et al. (U.S. Pat No. 4,713,224) describes a one-step process for removing minute quantities of impurities from inert gases, where the impurities are selected from the group consisting of carbon monoxide, carbon dioxide, oxygen, hydrogen, water and mixture thereof. The process comprises contacting the gas with a particulate material comprised of nickel in an amount of at least about 5% by weight as elemental nickel and having a large surface area, from about 100 to about 200 m2/g. There is no disclosure of removal of moisture from acid gases.
Tom et al (U.S. Pat Nos. 4,853,148 and 4,925,646) discloses processes and compositions for drying of gaseous hydrogen halides of the formula HX, where X is selected from the group consisting of bromine, chlorine, fluorine, and iodine. The patent describes the use of, for example, an organometallic compound such as an alkylmagnesium compound, on a support. The halide is substituted for the alkyl functional group. Suitable supports are, alumina, silica, and aluminosilicates (natural or synthetic). However, there is no description or suggestion of reducing or removing moisture from sulfur-containing compounds. Alvarez, Jr. et al. (U.S. Pat. No. 5,910,292) describes a process and apparatus for removal of water from corrosive halogen gases, using a high silica zeolite, preferably high silica mordenite. The patent describes removing moisture down to less than or equal to 100 ppb water concentration in halogen gases, particularly chlorine- or bromine-containing gases. U.S. Pat. No 6,183,539 discloses utilizing high sodium, low silica faujasite particles for the adsorption of carbon dioxide and water vapor from gas streams. The disclosed types of gas streams in which this type of high sodium, low silica faujasite crystals can be utilized includes air, nitrogen, hydrogen, natural gas, individual hydrocarbons and monomers, such as ethylene, propylene, 1.3 butadiene, isoprene and other such gas systems. There is no mention of sulfur-containing acid gas purification using the faujasites.
U.S. Pat. No. 4,358,627 discloses use of xe2x80x9cacid resistantxe2x80x9d molecular sieves, such as that known under the trade designation xe2x80x9cAW300xe2x80x9d, for reducing the chloride concentration in chlorinated liquid hydrocarbons that contain an ethylenically unsaturated chlorinated hydrocarbon, water and hydrogen chloride. The method includes providing certain nitrogen-containing compounds in the system and contacting the system with the molecular sieve. There is no disclosure or suggestion, however, of removal or reduction of moisture from gas phase compositions, or removal or reduction of moisture from liquids comprising acid gases.
Given the problem of moisture reacting with sulfur-containing acid gases, it would be advantageous if apparatus and methods could be provided which reduce or overcomes the problem.
In accordance with the present invention, certain acid gas resistant molecular sieve compositions are employed to reduce or remove moisture from fluid compositions comprising a sulfur-containing compound. As used herein the term xe2x80x9cremovexe2x80x9d means that the water content of the final composition comprising the sulfur-containing compound will be equal to or less than 100 parts per billion (ppb), more preferably less than 10 ppb, and more preferably less than 1 ppb. As used herein the term xe2x80x9creducexe2x80x9d means that the moisture concentration of the final composition comprising the sulfur-containing compound will be no more than 0.1 times the starting fluid composition water concentration, preferably no more than 0.01 times, and more preferably no more than 0.001 times the starting moisture concentration. Presently, the detection limit for moisture is about 4 ppm in sulfur-containing fluids. Compositions are made to 4 ppm concentration, then diluted to the desired reduced moisture concentration. As used herein the term xe2x80x9csulfur-containing compoundxe2x80x9d includes carbondisulfide, carbonylsulfide, and compounds within formula (I):
Yxe2x80x94Sxe2x80x94Xxe2x80x83xe2x80x83(I)
wherein S is sulfur,
X and Y are the same or different and are independently selected from the group consisting of hydrogen, alkyl, aryl oxygen, and alcohol.
Examples of preferred sulfur-containing compounds within formula (I) include hydrogen sulfide, sulfur dioxide, methylthiol, ethylthiol, n-propylthiol, i-propylthiol, benzylthiol, and the like.
A first aspect of the invention relates to an apparatus comprising:
a) a container having an internal space;
b) an acid gas-resistant molecular sieve positioned within at least a portion of the internal space; and
c) means for maintaining said molecular sieve within the container when a fluid comprising a sulfur-containing compound is caused to flow through the molecular sieve.
Preferred apparatus are those wherein the acid gas-resistant molecular sieve is selected from the group consisting of molecular sieves having an effective pore size ranging from about 1 Angstrom up to about 10 Angstroms, more preferably ranging from about 3 to about 8 Angstroms. Preferred are the molecular sieves known under the trade designations AW300 and AW500, particularly herein the molecular sieve is AW300 The molecular sieve is preferably positioned in the container so that substantially all of the sulfur-containing compound passes through the molecular sieve. Also preferred are apparatus wherein molecular sieve completely fills the internal space, and the means for maintaining the molecular sieve in the container is a material that is substantially inert to the sulfur-containing compound. Preferably, the means for maintaining the molecular sieve in the container is the molecular sieve material itself contacting an inner surface of the internal space. Also preferred are apparatus comprising an effluent conduit removably attached to the container, the effluent conduit adapted to route at least a portion of a moisture-depleted gas stream to a diode laser moisture measurement system.
A second aspect of the invention are fluid compositions comprising a sulfur-containing compound, the compositions having reduced moisture concentration, the compositions comprising at least one sulfur-containing compound having a sulfur-containing compound concentration, and water having a water concentration, the water concentration being no more than 0.1 times the concentration of the sulfur-containing compound. Preferred compositions are those wherein the water concentration is no more than 0.01 times the concentration of the sulfur-containing compound, more preferably no more than 0.001 times the concentration of the sulfur-containing compound. Preferred compositions of the invention are those wherein there is a single sulfur-containing compound having a sulfur-containing compound concentration of no more than 1 part per million (ppm), and wherein the water concentration is no more than 1 ppb.
Fluid compositions of the invention comprising a sulfur-containing compound and water vapor are preferably pure fluids of one acid gas, but the fluid may be a mixture of a single acid gas with another gas, such as an inert gas like nitrogen or argon. The fluid comprising acid gas may also comprise a mixture of two or more acid gases, either with or without another gas.
A third aspect of the invention is a method of reducing moisture content of fluids comprising moisture and a sulfur-containing compound, the method comprising the step of passing the fluid through the apparatus of the first aspect of the invention. Preferred methods within this aspect of the invention are those wherein the sulfur-containing compound is selected from the group consisting of carbondlsulfide, carbonylsulfide, and compounds within formula (I) above.
Preferably, processes within the third aspect of the invention are carried out at combinations of temperature and flow rate that will ensure water in the fluid will not freeze, and where the sulfur-containing compound will not decompose. Preferred temperatures range from just above 0xc2x0 C. to just below a temperature where the sulfur-containing compound will decompose. Temperatures below 0xc2x0 C. are disfavored because of the possibility of water freezing in the container, or in the molecular sieve pores, or both. Temperatures above the decomposition temperature are disfavored due to possible decomposition. It maybe possible, through increased flow rate (or decreased residence time) conditions in the container, to operate below 0xc2x0 C. or exceed the decomposition temperature briefly. Generally it is preferred to operate at reduced temperature, as the acid gas resistant molecular sieve materials seem to operate more efficiently at these temperatures.
Further aspects and advantages of the invention will become apparent by reviewing the description of preferred embodiments that follow.